1. Technical Field
This disclosure relates to an image forming apparatus, and more particularly, to an image forming apparatus including a latent image carrier rotated to bear an electrostatic latent image on a surface thereof, a charger to evenly charge the surface of the latent image carrier, a writing device to irradiate a charged surface of the latent image carrier with a light beam according to image data to form an electrostatic latent image on the charged surface of the latent image carrier, a writing device control unit to control the writing device, a developing device to develop the electrostatic latent image with a developer to form a toner image on the charged surface of the latent image carrier, a transfer device to transfer the toner image formed on the charged surface of the latent image carrier onto a transferred member, and a pre-transfer irradiating device to irradiate the surface of the latent image carrier during a period between development and transfer.
2. Description of the Background
Related-art image forming apparatuses such as electrophotographic copiers and electrophotographic printers typically form a toner image on a recording medium (e.g., a sheet) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of a latent image carrier (e.g., a photoconductor); a writing device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
Pre-transfer light irradiation is a well-known process performed during a period between development and transfer and using, for example, a pre-transfer lamp (PTL). Specifically, in pre-transfer light irradiation, light is directed onto the surface of the photoconductor having the toner image thereon during a period between development and transfer to reduce electric charge of the electrostatic latent image formed on the surface of the photoconductor below the toner image. Accordingly, a transfer current employed during transfer is able to flow efficiently to a transfer material, providing stable transfer efficiency from lower transfer currents to higher transfer currents. Further, in full-color image forming apparatuses in which toner images of multiple colors, such as yellow, magenta, cyan, and black, are sequentially transferred onto a sheet in a superimposed manner to form a full-color image on the sheet, performing pre-transfer light irradiation prevents toner of the toner images from being reversely transferred onto the surface of the photoconductor during transfer.
However, pre-transfer light irradiation may cause some problems. For example, in an image forming apparatus employing a reversal developing method, a photoconductor gains positive electric charge after transfer due to electric discharge during transfer regardless of whether or not pre-transfer light irradiation has been performed. In a case in which pre-transfer light irradiation has not been performed, an electric potential on a surface of the photoconductor does not exceed 0 V after transfer because an absolute value of the electric potential on the surface of the photoconductor before transfer is sufficiently large. As a result, a memory of an image formed on the photoconductor during a previous sequence of image forming operations can be erased before the surface of the photoconductor is charged again for the next sequence of image forming operations by performing irradiation after transfer as illustrated in FIG. 12A.
By contrast, in a case in which pre-transfer light irradiation has been performed, the electric potential on the surface of the photoconductor before transfer is around 0 V, so that the electric potential on the surface of the photoconductor becomes positive after transfer as illustrated in FIG. 12B. Further, during transfer, electric discharge tends to occur at a non-image portion of the toner image formed on the surface of the photoconductor compared with an image portion having toner thereon. As a result, the surface of the photoconductor tends to have a large positive electric charge at the non-image portion. Because speed of movement of electrons is considerably slow, an image memory having such a large positive electric potential cannot be erased even when irradiation is performed after transfer. Consequently, the memory of an image formed on the photoconductor during a previous sequence of image forming operations remains and causes an uneven image formed during the next sequence of image forming operations.
Published Unexamined Japanese Patent Application No. H05-289472 proposes provision of two chargers to erase such a positively-charged image memory. However, such an arrangement is costly and requires a large installation space.